Glow-discharge display device including cathode elements of finely divided carbon

ABSTRACT

A cross-bar-addressed dot-matrix glow-discharge display tube is constructed so that each discharge path has a current-voltage characteristic the slope of which is unusually high for lowcurrent densities at the cathode surfaces, this characteristic being similar to that obtained by connecting a resistor in series with a conventional discharge and thus enabling the discharges to exhibit &#39;&#39;&#39;&#39;storage&#39;&#39;&#39;&#39; properties. The increased slope of the characteristic is obtained by manufacturing the cathode surfaces from microscopically rough and/or finely divided carbon, possibly as an admixture with a semiconductor or insulator. The carbon may be provided on either an electrically conductive or an insulating substrate.

[ Sept. 23, 1975 11/1960 Snell, Jr. et 313/309 12/1969 Link et al. 313/311 10/1973 313/220 X ABSTRACT addressed dot-matrix glow-discharge disconstructed so that each discharge path pe of which g similar to that g the discharges properties. The increased slope of obtained by manufacturing the m microscopically rough and/or n, possibly as an admixture with a y be proy conductive or an insu- 7 Claims, 5 Drawing Figures Primary Examiner-Palmer C. Demeo Attorney, Agent, or Firm-Fr.ank R. Trifari; Carl P. Steinhauser A cross-barplay tube is has a current-voltage characteristic the S10 is unusually high for low-current densities at the cathode surfaces, this characteristic bein obtained by connecting a resistor in series with a conventional discharge and thus enablin to exhibit storage the characteristic is cathode surfaces fro finely divided carbo semiconductor or insulator. The carbon ma vided on either an electricall lating substrate.

. ..H01J 61/067 313/218, 346 R, 311, 210

GLOW-DISCHARGE DISPLAY DEVICE INCLUDING CATIIODE ELEMENTS OF FINELY DIVIDED CARBON [75] Inventors: Raymond Frederick Hall, Crawle John Michael Stuart Schofield, Relgate; James Smith, Horley; Jeffrey Charles Merrell Short, New Malden, all of England Assignee: U.S. Philips Corporation, New

York, NY.

Filed: Sept. 20, 1974 App]. No.: 507,722

Foreign Application Priority Data Sept. 28, 1973 United Kingdom...............

U.S. Cl. 313/210; 313/218; 313/346 Field of Search.........

References Cited UNITED STATES PATENTS United States Patent Hall et a1.

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GLOW-DHSCHARGE DISPLAY DEVICE TNCLUDIING CATHODE ELEMENTS OIF lFINELY DIVHDED (IAON This invention relates to an electrical glow-discharge display device comprising an array of glow-discharge cathode elements and an anode element adjacent each cathode element for co-operating therewith to define an array of direct current glow-discharge paths through a gas atmosphere contained in said device, the cathode elements being electrically interconnected in groups, as are the anode elements, so that an addressing system exists for said paths, the surfaces of the elements of each cathode group which contact said atmosphere being physically separate from each other.

If the cathode and anode elements are arranged in rows and columns the cathode element groups may each consist of a column of cathode elements and the anode element groups may each consist of a row of anode elements, the addressing system then being one form of cross-bar addressing system. (The terms row" and column are used herein merely in a comparative sense; in practice the array may be orientated so that the rows extend from top to bottom and the columns extend from side to side. In addition the rows may extend at an angle other than 90 to the columns. They may also be curved, as may the columns).

Such a device may be used for displaying simple patterns such as diagrams, numerals, or words. There are two modes of operation of such devices:

a. cyclic, when each discharge path is only energised during the period when the row and column containing that path are addressed, and

b. storage, when each discharge path is energised during an address period and remains on until such time as an off pulse is applied thereto. Once a discharge is energised it is necessary to control the current by means of a limiting resistor. In the cyclic mode such a resistor is connected in series with each anode row or each cathode column. The resistor is then time-shared between all the discharges on its row or column. Thus if the current limiting resistors are in the cathode columns the anode rows must be switched on in a cyclic manner..

If there are 11 rows a discharge cannot be operated for a time greater than a fraction 1/ n of the total field or frame time. (In practice the maximum operating time of each discharge will be even less than this because of its finite switch-on time). This mode of operation has the disadvantage that, while operating, each discharge must give out at least n times the mean light output required. The cycling time must also be sufficiently rapid to avoid a flickering effect from the display. I

The storage mode overcomes the duty-factor problem. it is theoretically possible because, once a discharge has been initiated, the voltage necessary to maintain that discharge is lower than that required to initiate it..Thus, if the row and column conductors are maintained at a relative potential difference V which is greater than the extinction voltage Ve for the dis charges but less than their strikingvoltage Vs, i.e. Vs V Ve, all elements will remain off until the voltage be tween the row and column conductor corresponding to a particular element is momentarily raised to above the striking voltage. When this is done the element in question will strike, and moreover it will remain on after the momentarily raised voltage has returned to its initial value. Thus successive elements can be energised in turn, and will remain on unless steps are taken to subsequently extinguish them. This can be done by momentarily reducing the voltage between the row and column conductor corresponding to a particular element.

There is one major problem which arises if energising a device in this way is attempted. The current-limiting resistors cannot be time-shared as in the cyclic mode of operation described above. One resistor must be per manently connected in series with each discharge path either in the anode or in the cathode lead thereto. This would be fairly easy to do if either all the individual anodes or, as in display tube type ZM 1251 obtainable under the Registered Trade Mark Mullard, all the individual cathodes were to have separate external connections, but this is not so in a panel including a crossbar addressing system and it is necessary then to include the individual resistors within the panel itself.

US. Pat. No. 3,764,847 discloses a construction in which the anode electrodes comprise electrically conductive films deposited in rows and columns on the inner surface of an optically permeable viewing window of the device, the anode films of each row being individually electrically connected to a common row supply conductor which forms part of a cross-bar ad dressing system, which is also deposited on said inner surface, and at least the major portion of which is covered with an electrically insulating layer, said electrical connections each being formed by an electrically resistive thin-film path deposited on said inner surface and covered with an electrical insulator.

It is an object of the present invention to provide an alternative construction in which the internal impedance of the discharge itself can be used to control the current'so. that in effect the current versus voltage characteristic is equivalent to that of known devices when an individual resistor is connected in series with each path. a

According to one aspect the invention provides an electrical glow-discharge display device comprising an array of glow-discharge cathode elements and an anode element adjacent each cathode element for cooperating therewith to define an array of direct current glow-discharge paths through a gas atmosphere contained in said device, the cathode elements being electrically interconnected in groups, as are the anode elements, so that an addressing system exists for said paths, the surfaces of the elements of each cathode group which contact said atmosphere being physically separate from each other and comprising carbon in finely divided form. The particles of carbon preferably have diameters lying in the range 0.01 am to 10 am.

According to another aspect the invention provides an electrical glow-discharge display device comprising an array of glow-discharge cathode elements and an anode element adjacent each cathode element for cooperating therewith to define an array of direct current glow-discharge paths through a gas atmosphere contained in-said device, the cathode elements being electrically interconnected in groups, as are the anode elements, sov that an addressing system exists for said paths, the surfaces of the elements of each cathode group which contact said atmosphere being physically separate from each other and comprising microscopically rough carbon.

It has been found that manufacturing the cathode surfaces which contact said atmosphere from carbon which is in finely divided form and/or which has a microscopically rough surface can result in a currentvoltage characteristic for each discharge path the slope dV/di of which is unusually high for low current densities at the cathode surface in a discharge cell in which no positive column occurs.

It should be noted that it is known that the positive slope dV/di Rg say, of such a gas discharge characteristic is inversely proportional to the cathode area and to the square of the gas pressure. However, in order to obtain values of Rg which are sufficiently high for the purpose referred to hereinbefore the required values of these parameters would be impractical with commonlyused cathode materials. In particular the low pressure and high current density implied would lead to rapid erosion of the cathode.

Because the sputtering rate of carbon is considerably lower than for many other materials a lower gas pressure and/or high current density can be tolerated when using this material. Nevertheless there is a practical lower limit to the pressure imposed by the erosion of even these cathodes and also by the possibility of the voltage characteristic of the discharge becoming unacceptable. The existence of a limiting lower value for the effective area of each cathode element if the required value (for example 400K ohms to 2M ohms) is to be realised for Rg. A suitable pressure value for pure neon has been found empirically to be 40-60 Torr for cathode element areas of approximately 0.4 mm and anode-cathode spacings of 1.3 mm. Rg may then lie in the range 700K ohms to 1M ohm.

The carbon may be in the form of carbon black", so-called baked carbon, colloidal graphite with potassium or sodium silicate, or so-called carbon paper" (available under the Registered Trade Mark Papyex). It may be present as an admixture with particles of a semiconductor or insulator such as silicon carbide or boron carbide respectively.

The carbon may be in the form of a coating or layer on an electrically conductive, e.g. metal, substrate. If this is the case the layer should be as thick as possible (consistent with good adhesion) to extend the life of the device. (Material is inevitably sputtered off in operation and it will be realised that immediately any part of the metal substrate is exposed the effect of the provision of the carbon will be lost and Rg will drop drastically, possibly resulting in destruction of the device). The coating or layer of carbon is therefore preferably provided on an electrically insulating substrate, in which case the coating or layer forming the surface of each cathode element of a group is preferably electrically connected to the other coatings or layers of that group by a further conductive material, such as a metal which may also be provided on said insulating substrate.

The beneficial effect of the provision of the carbon may be due to its lowering the electron secondary emission coefficient for the cathode surface. Electrons emitted by the cathode surface in response to bombardment by electrons or ions may tend to become trapped in the undulations of the surface, this being a well-known hypothesis to explain the low-electronelectron secondary emission coefficient of such surfaces.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which FIG. 1 shows glowdischarge current-voltage characteristics,

FIG. 2 is a plan view of part of a first glow discharge display tube;

FIG. 3 is a perspective view of a transverse section of the part of FIG. 2 together with a viewing window;

FIG. 4 is a longitudinal section of the part shown in FIG. 2 together with a viewing window; and

FIG. 5 shows separate parts of a second glowdischarge display tube.

In FIG. 1 curve A is the current i versus voltage V characteristic of a conventional glow-discharge path. It will be seen that the current initially increases slowly with voltage but eventually a point is reached beyond which the characteristic has a portion of negative slope. This point is the striking potential Vs of the path and, when it is just exceeded, the current increases rapidly until (in the absence of a current-limiting resistor) it reaches a point (not shown) on the slowly rising righthand part of the curve which corresponds to the applied voltage Vs. Because the right-hand part of the curve has such a shallow slope the final current will be large and may result in destruction of the device by an arc discharge.

Curve B, which may be obtained in a device according to the invention, corresponds to curve A at low currents but, in the region beyond the region of negative slope, it has a much steeper slope than curve A. Thus, if the discharge path has the characteristic denoted by curve B then, when the striking potential Vs is just exceeded, the current will only increase to a value i,,,, which is much lower than the current corresponding to Vs on the right-hand part of the curve A. This lower current need not result in damage to the device.

Vm denotes the minimum maintaining potential of the path, once a discharge has been struck. The maintaining potential corresponding to i, for curve B is Vs.

In FIG. 2 a block 1 of electrically insulating material, for example that available under the Registered Trade Mark FUSITE K, has been moulded around a set of cathode substrate strips 2 which are led through the sides of the block 1 in a vacuum tight manner. The upper surface of each substrate strip 2 is completely coated with a layer 12 of finely divided carbon, which may be 50 pm to m thick, except where the strips 2 pass through the side wall of the block 1 and also where they are exposed external to it. Said layers 12 may then be provided by spraying colloidal graphite with potassium or sodium silicate in a suitable liquid vehicle (obtainable under the Registered Trade Mark DAG) onto the substrates through a suitable mask using compressed air. After spraying, the layers are baked in vacuo, for example at lOO0C. The top face of the block 1 is provided with an array of cavities 3 which extend downwards as far as the coated strips 2. Thus a physically separate cathode surface is formed by the coated part of each strip 2 exposed at the closed end of each cavity. The cathode substrate strips may be made of the material available under the Registered Trade Mark KOVAR.

The top face of the block 1 is provided with a set of 1 parallel ridges 4 which extend in the length direction of the block 1 so as to form channels linking parallel rows of cavities 3. The cavities 3 are divided into parallel columns by grooves 5 provided in the top face of the block. These ridges and grooves can prevent sputtered material from the coated strip at the bottom of a cavity from forming a leakage path between one cavity and the next. Further coated cathode substrate strips 6 which are similar to the strips 2 are provided at each end of the array. The whole of these coated strips 6 is exposed except where an (uncoated) part thereof passes through the wall of the block 1.

FIG. 3 is a perspective view of a cross-section taken on the line III-III of "FIG. 2. A transparent viewing window plate 9 rests on the top of the ridges 4. This window may be made of material available under the trade name SOVERIL 805.51 and carries a set of parallel anode wires 10 moulded into the lower face thereof. These wires 10 are aligned with the rows of cavities 3 so that an anode element faces each cavity and hence each cathode element. The wires are led through the sides of the window 9 in a vacuum-tight manner. They form a cross-bar addressing system for the resulting array of glow-discharge paths in conjunc tion with the cathode substrate strips 2. The window 9 is sealed all round to the block 1 as at 11 in a vacuumtight manner. The anode wires 10 may be made of the material available under the Registered Trade Mark KOVAR".

FIG. 4 is a longitudinal section of the part shown in FIG. 2 taken on the line IVIV. It will be seen from FIG. 4 that the top face of the block 1 is provided with a step at each end. The auxiliary cathode substrate strips 6 lie on the face of this step and are consequently situated substantially in the same plane as are the substrate strips 2 and are thus each in a further channel extending in the column direction. The channels formed between the ridges 4 open into these further channels, all channels being closed by the plate 9 to form ducts. FIG. 4 also shows one anode wire 10.

In a practical embodiment the cavities 3 were disposed in 8 X 50 rectangular array. Each cavity was 0.75 mm square and the spacing between the cathode element surface 12 at the bottom of each cavity and the corresponding anode wire was 1.25 mm. The cavity depth was 0.75 mm and the ridges 4 were 0.6 mm high. The cavities were 1.5 mm between centers. The anode wires were in the form of strips 0.3 mm wide and 0.1 mm thick. The grooves 5 were 0.25 mm deep.

After closure the interior of the tube was evacuated through an exhaust tube (not shown) and was filled with a gas atmosphere of, for example, pure neon at 40-60 Torr.

The complete tube was then aged by applying an operating potential of 350 volts between each cathode and anode element for a short time.

The subsequent application of an operating potential of at least 280V across each discharge path was found to result in the generation of optical radiation from the negative-glow region in the resulting glow-discharge, the positive column part of the discharge being substationally or completely suppressed. The current-voltage characteristic of each discharge path was substantially as curve B of FIG. 1, i,,, being approximately 100 .LA.

In operation a steady dc. bias lying midway between Vm and Vs was applied between all the anode wires 10 and cathode substrate strips 2, and selected glow discharge paths in the cavities 3 were successively addressed by simultaneously applying a positive pulse to the corresponding anode wire 10 and an equal-value negative pulse to the corresponding cathode substrate strip 2, these pulses being of sufficient amplitude so that the striking potential Vs in the selected paths was just exceeded. Because of the current-voltage characteristics of each path, a discharge continued therein, once it has been addressed, until the potential across it was reduced below Vm by the application of pulses of opposite polarity to the corresponding anode wire 10 and substrate strip 2. A discharge was maintained to the cathode substrate strips 6 continuously and it was found that these continuous discharges gave rise to sufficient ionisation throughout the whole of the tube to prime the addressed discharge paths in the cavities 3.

FIG. 5 shows an exploded. view of a second glowdischarge display device which comprises an electrically insulating cathode substrate plate 13, an electrically insulating apertured intermediate plate 14 made, for example, from aluminum sheet which has been anodised to give it an electrically insulating skin, a spacer 15 into which plate 14 is located and an electrically insulating transparent viewing window plate 16.

The apertures 18 in the plate 14 are arranged in a rectangular array and a cathode element rectangle 17 of finely divided microscopically rough carbon is situated on the substrate 13 in register with each. The eath ode rectangles of each row are electrically interconnected by means of a metal strip 19 offset from the apertures. Electrically conductive anode strips 20 are provided on the underside of the anode plate 16, a strip 20 being in register with each column of apertures 18. The strips 19 and 20 thus form a cross-bar addressing system for an array of glow-discharge paths each defined by a cathode element 17', an aperture 18, and the surface element of an anode strip 20 which faces that aperture 18.

The components 17 and 19 may be provided on the substrate 13 by first screen printing the strips 19 (which may be of silver) and then spraying on the cathode rectangle 17 in the form of colloidal graphite with potassium or sodium silicate in a suitable liquid vehicle through a suitable mask in a manner similar to that described for the layer 12 in the device of FIGS. 24, the material of each rectangle 17 being for example 25 ,u.m to um thick and positioned so that it overlaps the corresponding strip 19 slightly and thus electrically contacts it. Alternatively the carbon can be made into a suitable ink and screen-printed in position by standard thick-film techniques. After the provision of the rectangle 17 the substrate is baked, for example at 500C.

The anode strips 20 may also be silver, screenprinted onto the window plate 16.

The plate 14 may be 0.6mm l.2mm thick, the spacer 15 being 25-50 ,u.m thicker than plate 14 to give a small gap between plate 14 and window 16. The apertures 18 may be 300 ,um in diameter. The widths of anode strips 20 should preferably not exceed ,um, in order that they should not unduly obscure any glowdischarge occurring in the underlying apertures 18 in operation.

After provision of the various layers 17, 19, 20 the peripheries of the components 13, 15 and 16 (which may all be of glass) are sealed together in a vacuumtight manner by means of a solder glass, and the interior evacuated through a pump stem (not shown) and filled with a glow-discharge atmosphere such as pure neon at 40-100 Torr. The resulting array of glowdischarge paths again had a voltage-current characteristic similar to curve B in FIG. 1.

The plates 13 and 16 may be slightly larger than the plate 14 and spacer 15 in order to facilitate electrical connection from an external circuit to the strips 19 and 20.

The spacer 15 may be dispensed with if protruberances of, for example, insulating material are provided on the window 16 or plate 14 in order to give the required small gap between them.

Although the anode and cathode elements in the embodiments described have been arranged in rows and columns, it will be evident that other forms of coordinate array may alternatively be used. For example a polar array may be employed with the interconnected cathode elements lying on concentric circles and the interconnected anode elements lying on radii of these circles.

In Both of the described embodiments the carbon of the layers 12 or 17 may be present as an admixture with particles of a semiconductor or insulator such as silicon carbide or boron carbide respectively.

What is claimed is:

1. An electrical glow-discharge display device comprising a hermetically sealed envelope containing an ionizable medium and a transparent window portion, an array of glow-discharge cathode elements and an anode element adjacent each cathode element for co operating therewith to define an array of direct current glow-discharge paths through the ionizable medium contained in said device, means interconnecting the cathode elements and the anode elements, respectively into groups forming an addressing system for said paths, each of said cathode elements having a substrate portion and a current-limiting surface layer portion in contact with the ionizable medium and physically separate from each other surface portion, said surface layer portion comprising carbon in finely divided form.

2. A device as claimed in claim 1 wherein the particles of carbon have diameters lying in the range 0.01 ,u.m to 10 um.

3. A device as claimed in Claim 2, wherein the carbon is admixed with particles of a semiconductor or insulator.

' 4. A device as claimed in claim 1, wherein the substrate is electrically conductive.

5. A device as claimed in claim 4 wherein the substrate of each cathode element of a group is a metal strip which extends to also form the substrate of the other cathode elements of that group.

6. A device as claimed in claim 1, wherein the substrate is electrically insulating.

7. A device as claimed in claim 6, wherein the current-limiting surface layer of each cathode element of a group is electrically connected to the other layers of that group by a further conductive material provided on said substrate. 

1. AN ELECTRICAL GLOW-DISCHARGE DISPLAY DEVICE COMPRISING A HERMETICALLY SEALED ENVELOPE CONTAINING AN IONIZABLE MEDIUM AND A TRANSPARENT WINDOW PORTION, AN ARRAY OF GLOW-DISCHARGE CATHODE ELEMENTS AND AN ANODE ELEMENT ADJACENT EACH CATHODE ELEMENT FOR CO-OPERATING THEREWITH TO DEFINE AN ARRAY OF DIRECT CURRENT GLOW-DISCHARGE PATHS THROUGH THE IONIZABLE MEDIUM CONTAINED IN SAID DEVICE, MEANS INTERCONNECTING THE CATHODE ELEMENTS AND THE ANODE ELEMENTS, RESPECTIVELY INTO GROUPS FORMING AN ADDRESSING SYSTEM FOR SAID PATHS, EACH OF SAID CATHODE ELEMENTS HAVING A SUBSTRATE PORTION AND A CURRENT-LIMITING SURFACE LAYER PORTION IN CONTACT WITH THE IONIZABLE MEDIUM AND PHYSICALLY SEPRATE FROM EACH OTHER SURFACE PORTION, SAID SURFACE LAYER PORTION COMPRISING CARBON IN FINELY DIVIDED FORM.
 2. A device as claimed in claim 1 wherein the particles of carbon have diameters lying in the range 0.01 Mu m to 10 Mu m.
 3. A device as claimed in Claim 2, wherein the carbon is admixed with particles of a semiconductor or insulator.
 4. A device as claimed in claim 1, wherein the substrate is electrically conductive.
 5. A device as claimed in claim 4 wherein the substrate of each cathode element of a group is a metal strip which extends to also form the substrate of the other cathode elements of that group.
 6. A device as claimed in claim 1, wherein the substrate is electrically insulating.
 7. A device as claimed in claim 6, wherein the current-limiting surface layer of each cathode element of a group is electrically connected to the other layers of that group by a further conductive material provided on said substrate. 